Suppose crime scene investigator's come upon document such as a ransom note or a suicide note that has been copied in an endeavor to hide the offender's handwriting, the ink he used, or typewritten text. How can they decide that this copied piece of paper came from a particular copy machine?
In this article, I will endeavor to by comparison how forensic document analysts decide a link in the middle of a paper copy to the engine that produced the copy.
A copy engine has the capability to duplicate an image from one piece of paper onto another straight through a complicated series of events. First of all, a lens focuses the image of the former paper onto what is known as a drum that is expensed up with electrons and coated with selenium or some other substance that is sensitive to light. The image is then retained on the drum as the drum is coated with toner powder. The toner powder sticks to the exterior of the drum in direct relation to the degree of strength of the electrostatic charge of the electrons. This toner image is then transcribed onto a blank piece of paper (or copy) which, in turn, is exposed to a fixing agent. A fixing agent is a chemical used to "attach" the toner powder permanently on the blank piece of paper.
For the purposes of forensic document analysis, criminal investigators can sometimes link a photocopied document to a particular copy engine by creating a comparison document to correlate the photocopy against. This is due to the fact that the mechanisms inside the engine that pull the paper onto and remove it from the copy exterior leave distinctive markings on the paper. Similarly, the camera lens, cover glass, or drum exterior may have scratches, etches, or abnormalities that mark every copy that it creates. From time to time, these markings show up on copied paper. Forensic scientists can then recognize and link this marked copy to the copier that created it and perhaps the perpetrator to the crime.
With advancing technology, forensic scientists have many tools in its bag of tricks to link a think to a crime.
